1. Field of the Invention
The present invention relates to a method of producing an aqueous polymer emulsion having excellent stability. More specifically, the present invention is intended for producing an aqueous emulsion containing substantially no emulsifier and relates to a method of producing an aqueous polymer emulsion remarkably elevated in chemical stability by copolymerizing a specific amount of a particular vinyl monomer containing polyoxyethylene units as an indispensable copolymerization component.
2. Description of the Prior Art
In order to maintain the stability of aqueous polymer emulsions, it has been generally the practice heretofore in producing such emulsions to add a low molecular weight emulsifier upon polymerization or to compound a low molecular weight emulsifier or dispersant after polymerization. In recent years, there is an increasing demand in using aqueous polymer emulsions for paints, cement or mortar mixing, adhesives, textile treatment and paper processing. For such applications, the emulsion in intact form is usually used and not the polymer itself removed from the emulsion. Accordingly, when the emulsion contains an emulsifier or dispersant, foaming of the emulsion will occur on account of the emulsifier, etc. contained, and this makes it handling upon processing extremely difficult. Of course, the addition of a defoaming agent is attempted to prevent foaming, but when such an emulsion containing a defoaming agent is used for the above-mentioned purposes such as paint or cement, the mechanical strength or moistureproofing of the film or shaped product will be seriously lowered by the secondary action of the emulsifier or defoaming agent, and this will constitute a great hindrance in practical use.
In order to obviate the defects attendant on such emulsifier-containing emulsions, aqueous emulsions free from low molecular weight emulsifiers have been noted in recent years, and many researches are being made of such emulsifier-free aqueous polymer emulsions or methods for producing the same.
The emulsifier-free aqueous polymer emulsions have various excellent features in comparison with the conventional emulsifier-containing emulsions. But on the other side, they involve a defect such that they are remarkably low in chemical stability, particularly in the stability against electrolytes, in comparison with the usual emulsifier-containing emulsions, and easily precipitate or coagulate by the addition of a small amount of electrolytes. Such defects present various problems and pose difficulties in practical use for applications wherein large amounts of electrolytes and other substances are mixed, as in paints, cement or mortar mixing, adhesives, paper processing and textile treatment.
To improve the chemical stability of emulsifier-free aqueous emulsions, a method is proposed in Japanese KOKAI Pat. No. 51-30284 wherein a monomer represented by the following general formula is copolymerized as a copolymerization component: ##STR1## wherein R.sub.1 is H or an alkyl group of C.sub.1 to C.sub.4 ; R.sub.2 is H or an alkyl group of C.sub.1 to C.sub.22 ; 5.ltoreq.m.ltoreq.150; R.sub.3 is H in which case n=1 to 3 or R.sub.3 is a methyl group in which case n=2; M is a monovalent cation such as an alkali metal or ammonium base. When such a monomer is used, the chemical stability will be improved to some extent, but it is not good enough. Moreover, because the monomer is extremely low in radical polymerization activity, the polymerization reaction requires a long time and the reaction ratio is low. On account of these disadvantages, this method is difficult to be employed on a practical basis. In addition, it is a defect that the process for producing said monomer is complicated.